Bearing Capacity Calculation 1

[PEandCat]

Hi there,

It's been over 7 years since my geotechnical class, please forgive me for my silly questions:

Q1: There are more than one equation available for calculation bearing capacity in textbooks? Is the Terzaghi's formula the most commonly used, or how do you determine which one is the most appropriate?

Q2: The ultimate bearing capacity equation involves phi' and c' (or Su for clay?). What tests should be performed to obtain these parameters for sand, and for clay respectively?

Q3: Should the parameters in the equation always be under the drained conditions, so are the tests performed to obtain the parameters?


Thanks in advance for your helP and patience.


ymchan2

 

[fattdad]

ultimate bearing capacity, even safely factored, rarely determines the design bearing pressure. Usually forecasted settlement governs.

Read some good books, including DM-7.1 and 7.2 for further insight on the rational methods for bearing capacity determination. There are correlations to SPT N-value also. It's quite hard to answer the "best" method without some context (i.e., regional geologic setting and local conventions).

f-d

¡papá gordo ain’t no madre flaca!

 

[iandig]

It can also depend on which country and which standards you are working to. If following Eurocodes then they now tell you which equations you should be using under the Eurocoode 7 documentations whihc is issued as standards refenced with the EN 1997-1, EN 1997-2 (and EN 1997-3 if outside of the UK). There is no great difference between what most engineers were using previoulsy, it has just become codified into a standard to be applied across Europe.
The prefix for each of these standards reflects teh Country/language used, i.e. in the UK it will be BS EN 1997, in Germany it is DIN EN 1997.
There is a real advantage to this, as it links the investigation and testing you have to do to the equation to be used in the design. Therefore when you specify investigaiton, sampling and testing it can be directly linked through the standard to how the data will be handled.
You can still use any equation you feel comfortable with but the standard requires that you provide a justification for it. It also defines three classes of structures to be built, and links what you have to do to the complexity of the design. Big changes in using these are that more 'traditional' methods of sampling, testing and calculating are no longer appropriate for more complex structures.

 

[BigH]

Ian - and what happens when you follow the code "religiously" and the structure/foundation fails . . .? I went into geotechnical engineering because it wasn't codified and one was really allowed to use one's ingenuity to solve problems. With codification, they think that they have it all licked, now.

 

[iandig]

Ah well, I guess I probably made it sound like it was all clearly defined and there was no scope for good engineering.
It does depend on the structure to be built but also when you go through the appendicies they are direct copies out of the text books 'we' have always used, presented as 'examples of how... can be determined'. I think I missed the point i was trying to make, which was IF you are in a Country which uses the EN system, then the standard identifies appropriate sources of information and methodology to use. For example when interpretting plate load tests to assess the plate settlement modulus, it refers directly to Burland (1969). Lots of the 'examples' refer back to Tomlinson, Burland, Bergdahl, US Army Corp of Engineers, etc... It forms a good starting point to research what options you have in certain cricumstances but does not limit the Engineer to use their judgement.
As mentioned above, the real difference (for the UK at least) is that samples taken with a U100 liner, or in alluminum U tube are now clearly limited as to what quality of specimen they are and what there appropriate use is.

 

[PEandCat]

Hi all,

Thanks for the reply. I am in the US, and I am just asking these questions in the general sense, no particular project in mind. It seems like Question #1 was too broad to be answered.

Would you mind shedding some lights on questions #2 and #3 though?

Q2: The ultimate bearing capacity equation involves phi' and c' (or Su for clay?). What tests should be performed to obtain these parameters for sand, and for clay respectively?

Q3: Should the parameters in the equation always be under the drained conditions, so are the tests performed to obtain the parameters?


Thanks so much!!!

ymchan2

 

[BigH]

Q2: Yes. For granular soils (sands, gravelly sand, etc) you will use phi'. c' will be zero. Obtain estimate of phi' from SPT test and Terzaghi or other correlations. To obtain phi' by triaxial test or direct shear test, it will be difficult to "make" the sample to match the undisturbed state and texture as in the field. It is extremely difficult to obtain an undisturbed sample of sandy material. For clays, you can obtain Su (which would be taken as c') in the equations from such direct tests as the vane shear, CPT (with some correlation of Nk). If you can obtain an undisturbed sample (block sample being the best, large diameter thin-walled steel tube the next choice - preferrably obtained by piston push), you can do quick triaxials, unconfined compressive strength (not as good), direct shear, etc. Note that Su will vary according to orientation - direct shear is preferable for the "bottom" - flat lying - and triaxial for those failure surfaces on an incline (see Lambe and Whitman).
Q3 - for sands and heavily overconsolidated soils, the drained analysis would be used (why for overconsolidated clays - because with the suction of water to counter the negative pore pressures developed on loading, the strength of the clays will decrease with time). For normally or slightly overconsolidated clays, you would use the undrained analysis (substituting Su for c'). If you could determine the actual porepressure response to loading, you could use the drained analysis but would need to be able to incorporate the porepressures in the analysis.

 

[iandig]

There was a very good Technical Paper in the Ground Engineering magazne in Fenruary of this year which looked at the use of the Plate Load Test to back calculate the angle of shearing resistance using a notional failure settlement of the plate to determine the ultimate bearing capacity. Then from this starting point, back-calcualting what the equivalent angle of the material would be 9for the size and shape of the plate used) for this ultimate bearign capacity.
The use of this was being proposed for the design of working platforms for tracked plant. The paper suggested that a settlement of 10% of the plate radius was appropriate in most instances. of note was that he angle returned from this paper was generally higher than a correlated SPT N value, a less conservative approach but a specific assessment of the actual ground in question.
I have not used this approach in the design of permenant foundations, but I have found it very useful in instances where there is a limit of information whihc has been provided for review. Tou also need to recognise that the zone of influence from a plate test is limited.

 

[BigH]

Ian - I don't look at the plate load test to be anything better than to confirm pavement moduli - why? because of the zone of influence problem . . .

 

[iandig]

The technical paper was 'purely' for working platform desin, no more than 2m required in accordacne with BRE BR470 if theunderlying soils are consitent.